1. Field of the Invention
The present invention relates to a digital electronic still camera in which image signal data is compressed to be recorded in a recording medium such as a memory card.
2. Description of the Related Art
In an electronic still camera a memory card is detachably connected to a camera body having an image pickup optical system, a solid state pickup device, or the like, and image information of a still image picked up by such an image pickup device is stored in the memory card in the form of digital signals. The memory card storing the image information is taken out of the camera and set in a reproducing device to reproduce the still image read out thereof on a television screen. The image information stored in the memory card also is transmitted to and stored in a large capacity memory such as a hard disk so that it may be an electronic album. Further, the image information is transmitted through a data communication line by connecting the memory card to a communication system so that image data communication may be achieved.
Memory cards currently in actual use are those with about 16 Mbit semiconductor memory of SRAM. It has been desired to record as many still images or still pictures and sounds in the memory card as possible, which has led to development of compression of the recorded information. Adaptive discrete cosine transformation (ADCT) is a standardized basic method for the image data compression. Discrete cosine transformation is described in N. Ahmed, T. Natarajan, and K. R. Rao: "Discrete Cosine Transform", IEEE Transactions on Computers, pages 90-93 (January, 1974).
In case that the electronic still camera is used as a scanner scanning a black board like an electronic board to store information thereon, letters or characters are dominant in the photographed information. Conventionally, such an image dominated by letters has been compressed by the same method as that which is used for image of landscape or portrait which is hereinafter called an image of nature. The compression method applied to the image of nature is inefficient to compress the letter image due to its high redundancy.
This unsatisfactory compression of letter images in turn results in disadvantageous use of memory capacity of the memory card, since limited is the total amount of information to be recorded in the recording medium or memory card of electronic still camera.
In the conventional electronic still camera, a quantization table is used upon quantizing data as means to control a quality of an image. But an optimum quantization table has never been used separately for each kind of images, i.e., letter image, nature image, etc.